Grid



Dec. 7, 1937.

H. VARIAN GRID I Filed March 15, 1935 VAR/AN.

R w my u L m Ono M! w MATERIAL OF HIGH 4 MAGNET/Cv PERMEABlL/TK ATTORNEY I COLLOID/IL GRAPH/TE.

INSULATING MATER/AL.

Patented Dec. 7, 1937 UNE'E'ED STATES GRID Russell H. Varian, San Francisco, Calif., assignor to Farnsworth Television Incorporated, a corporation or California,

Application March 13, 1935, Serial No. 10,892

1 Claim.

My invention relates to thermionic tube grids, and more particularly to such grids as may be used in tubes adapted for operation at high frequencies in any of the various manners well known in the art.

The principal object of my invention is to prevent, in a large degree, the acquisition of oscillating potentials in grids of tubes handling high frequencies, and particularly to prevent such potentials from occurring in grids carrying a steady D. C. biasing potential, sometimes called space charge grids, the objective to be obtained being that the inter-electrode capacities may thus be greatly reduced.

Other objects of my invention are: to provide a thermionic tube grid having a relatively high resistance to high frequency alternating currents; to provide such a grid which will also have a relatively low resistance to direct current; to provide a grid of the character described having a relatively high magnetic permeability; to provide such a grid which will have a high skin effect; and to provide a grid which will resist influences tending to change its potential when operated in a tube carrying high frequency alternating currents.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claim.

In the drawing which represents, schematically, several embodiments of my invention:

Figure 1 is a longitudinal section of a thermionic tube having an anode, two grids, and a cathode, the grids being in the form of inductances.

Figure 2 is a sectional view taken in the plane as indicated by the line 2-2 in Figure 1.

Figure 3 is a longitudinal sectional view of the elements only of a tube containing elements similar to those shown in Figure 1, except that the turns of the grid are shorted by supporting members.

Figure 4 is a sectional View of a grid wire formed. from a material having high magnetic permeability.

Figure 5 is a cross sectional view of a grid wire having a roughened surface.

Figure 6 is a cross sectional View of a grid element comprising an insulator covered with high resistance material.

In broad terms, my invention comprises making the grid of a thermionic tube resistant to sud.-

den changes in potential during operation of the tube at high frequencies. As there are a number of ways in which this particular resistance may be embodied in a thermionic tube grid, I believe my invention may be more thoroughly understood by reference to the drawing. This drawing shows, without any attempt to show actual practical construction, the general arrangement of electrodes as used in present-day thermionic tubes. It should be understood, however, that the invention may be applied to tubes having Various numbers of electrodes cooperating in other ways than that shown. The application of my invention to such other thermionic tubes will be readily apparent to those skilled in the art.

In the figures above referred to, an envelope l is provided with a reentrant stem 2 carrying a lead seal or press 3. A central cathode ,4, in this case a filament, is supported by cathode leads ,5 which pass through the press, and by a filament support 6 which is extended upwardly beyond the bight l to enter and support an insulating block 8. I prefer to form a slight bend 9 in the filament support 6 on which the bight I may rest. An inner grid l!) is supported at one end by an inner grid lead H, and at the upper end by an inner grid insertion l2 entering the block 8. An outer grid I3 is supported at the lower end by an outer grid lead Hi, and at the upper end by an outer grid insertion l5 fastened also to the block 8. No longitudinal supports are used for these grids, therefore they are each in the form of a continuous spiral and will be inductive. Both of these grids and the cathode are surrounded by an anode it, supported on an anode lead H, and an anode dummy it.

The embodiment shown in Figure 3 is identical with that shown in Figure l with the exception that no insulating block is used, and the grids are supported by their leads and corresponding opposing dummies l9 and 2c, the lead and dummy in this example passing longitudinally along the grid for the full length thereof, each turn of the grid being welded thereto. The grids, therefore, in this case are not inductive, but their structure is such that at relatively high frequencies loops 2i formed by the grid wires may well be of such size in relation to the frequency of the tube that they will tend to carry circulating currents and consequent oscillating potentials.

Before going into the material and general structure of the individual grid wires, it may be well to point out that the effective capacity of a grid is a function of its spacing from other electrodes, and the potentials between that grid and the other electrodes. Constant potential biases are customarily used on many of the grids and it may be that in a tube of the type shown the inner grid will be used as a control electrode having a varying potential thereon, whereas the outer grid may be used as a space charge grid with a. constant potential impressed thereon. It would appear, therefore, that it is extremely important that the potential on the outer grid should not vary, because if potentials are formed on this outer grid during the operation of the tube at high frequencies, which depart from the steady potential deliberately placed thereon, it

is plain to be seen that the operation of the tube may be seriously affected by the extraneous potentials, and the eifective operating capacity raised. In order to prevent such a space charge grid from varying in potential during the operation of the tube, and yet maintain the bias thereon constant, I form the grid of a material which has a relatively high frequency resistance. A convenient method of providing this high resistance is to make the grid of material having a high skin effect.

. While this-high skin effect may be obtained in a number of ways, I have shown three particular embodiments wherein this effect can be obtained. V i

The first comprises roughening the surface of the grid material; as shown in Figure 5. As the high frequency currents tend to flow on the outside of the conductor, such a roughening will tend to increase the high frequency resistance without greatly altering the resistance to direct current.

In the embodiment shown in Figure 6, the main body of the grid is formed of an insulating material, such as glass fibres, having a coating therer on of a relatively high resistance material, such as colloidal graphite. Here, of course, the D.- C. resistance is raised somewhat but not nearly to the extent as is the high frequency resistance.

As material of high magnetic permeability has a high skin effect, and consequent high resistance toward high frequency, I may desire to make the grid wires of such material and also may desire to roughen the surface of this material to add to the skin effect, thus combining the eifects.

It should be noted that in Figure 1 the entire grid is in the form of an inductance. The inductance is made such that it will not resonate with the frequencies normally handled by the tube. There is a long path provided for high frequency currents which greatly aids the skin effect in reducing circulating currents with consequent differences of potential. In the form shown in Figure 3, where the grid wires are connected by the supports, there are, of course, paths for circulating currents, but as the paths may be made to have an extremely high resistance to high frequencies the circulating currents are prevented from reaching an amount where the potentials associated with them are sufficient to substantially raise the inter-electrode capacities.

Other means of increasing the high frequency resistance of the grid will be apparent to those skilled in the art, and the requirements of my invention will be fulfilled if the high frequency resistance of the grid be made sufiiciently high to substantially prevent alternating potentials on the grid, due to the operation of the tube at high frequency, while still allowing the constant potential to be carried by the entire grid structure during the operation of the device, thereby preventing substantial increase of interelectrode capacities during operation at high frequencies.

lclaim: 1

A thermionic tube grid having a relatively high resistance to high frequency alternating currents and a relatively low resistance to direct current comprising a non-conducting vitreous foundation and a surface layer of graphite.

RUSSELL H. VARIAN. 

